A multi-layer printed circuit board is intended to be understood to mean a printed circuit board which has a plurality of planes which are situated vertically one above the other and which are each fitted with conductor tracks. The conductor tracks which are arranged on the various planes can be electrically connected to one another by means of so-called vias. These vias, also called electrical plated through-holes, are generally realized by a vertical bore which is metallized over its inside diameter.
By way of example, DE 10 2008 062 575 A1 discloses a multi-layer printed circuit board. Said document discloses a linear motor, the primary part of which is in the form of a multi-layer printed circuit board. The various layers of this multi-layer printed circuit board are for the most part filled with turns which can be energized. Turns which are situated one above the other in each case form a coil of a phase of the linear motor. A particularly lightweight and compact primary path for a linear motor, which primary path is particularly suitable as a rotor for highly dynamic applications, can be realized in this way.
FIG. 1 shows an arrangement of flat coils 1-6 for forming a solenoid coil in a cross section 8 perpendicular to the surface of a multi-layer printed circuit board. In said figure, by way of example, a flat coil 1-6 with in each case three turns which run in a spiral manner either from the inside to the outside or from the outside to the inside is located in each of the planes of the multi-layer printed circuit board. By way of example, a first flat coil 1 is wound from the outside to the inside and is electrically connected to a second flat coil 2, which is situated beneath it, by means of an electrical via. The second flat coil 2 is, in turn, wound from the inside to the outside in a spiral manner and for its part is once again connected to a third flat coil 3 in the third plane of the multi-layer printed circuit board by means of a further electrical via, not illustrated here. A solenoid which extends over six planes of the multi-layer printed circuit board is created in this way.
Multi-layer printed circuit board technology is also particularly suitable for realizing applications with high electrical power in a compact and lightweight design. An example of this is the abovementioned primary part of the linear motor from DE 10 2008 062 575 A1 which is realized as a multi-layer printed circuit board. Owing to the high currents in applications of this kind, the removal of heat is a particular challenge here. Therefore, it is necessary, for example—as illustrated in FIG. 1—to route the heat produced in the inner conductor tracks laterally to the outer printed circuit board edge where it can then be discharged to the surface of the printed circuit board. An insulating gap which can be of the order of magnitude of a few hundred micrometers is to be provided between the individual conductor tracks of a turn of each flat coil. The electrical insulation is ensured, for example, by prepreg layers, but the thermal conductance of said prepreg layers is low. Accordingly, a particular challenge is to discharge the heat from the central region of a multi-layer printed circuit board of this kind.